Various kinds of rotary welding members made of synthetic resin material of the type mentioned above have been proposed (for example, Japanese Patent Publication No. SHO 57-36131, and Japanese Utility Model Public Disclosure Nos. SHO 60-62613 and 60-155654).
A conventional welding member made of resin material for being rotated and welded to a resin surface such as, for example, a stud 1 (FIG. 1) and fastener 2 (FIG. 2) usually has such a structure as that of a post portion 3 with a disk-shaped flange portion 4 mounted upon one end thereof, and the flange portion 4 is provided with engaging projections 5 projecting from the peripheral portion at opposite sides of a welding surface thereof.
In order to mount this member made of resin on a mounting base plate madde of resin, the stud 1 or the fastener 2 is, as is shown in FIG. 3, held and pressed at the desired position on a mounting base plate 6 made of resin by means of a rotary jig 7, and a cut-out section 7a of the rotary jig 7 is engaged with each engaging projection 5 and rotated at a high rate of speed (FIG. 3). Both contacting surfaces of the flange portion 4 and the mounting base plate 6 are melted by means of frictional heat generated at that time. After the rotation has been stopped, the melting portion cools and hardens. As a result, the stud 1 or the fastener 2 is firmly secured to the mounting base plate 6.
However, in the stud 1 or the fastener 2 system of the above-described structure, the contacting surfaces of the flange portion 4 and the mounting base plate 6 are melted by means of frictional heat generated when the rotary welding process is performed. In addition, if the mounting base plate 6 is not thick enough, even the rear surface of the mounting base plate 6 is softened by means of such frictional heat. As a result, as is shown in FIG. 4(a), the rear surface of the mounting base plate 6 is expanded outwardly by means of the pressure of the rotary jig 7 against the stud 1 or the fastener 2 for performance of the welding process, or as is shown in FIG. 4(b), the rear surface of the mounting base plate 6 is warped or wrinkled because of the difference in the coefficient of contraction between the flange portion 4 and the mounting base plate 6 which are melted and welded together and then cool and harden at different rates or to different degrees of amounts, the difference resulting from the difference in the coefficient of linear expansion between the flange portion 4 and the mounting base plate 6. In other words, so-called " set-off" occurs.
Because of the foregoing, in the case where the rear surface of the mounting base plate 6 constitutes an outer surface of a product, the appearance is significantly spoiled by this means of this set-off phenomenon.